This invention relates generally to a method and apparatus for amplifier biasing. More specifically, this invention relates to a method and apparatus for optimizing the temperature dependence of various performance parameters of an amplifier.
The design of a typical bipolar amplifier requires trade-offs between a number of characteristics, such as noise voltage, gain, and output quiescent voltage over a range of various environmental and process conditions. Typically, designs which require low noise must use an undegenerated common-emitter amplifier (or differential pair for differential designs) with resistive loading. An example of such an amplifier is shown in FIG. 1 which depicts an amplifier stage 100 including a transistor QI fed at the base by a voltage VIN and connected at the collector to a supply voltage VCC, via a resistor R. Connected to the emitter of the transistor is a current source IEE. The output voltage is given as VOUT=VCC-ICC*R, and the gain is roughly given by AV=IEE*R/VT, where VT=kT/q, and k is Boltzmann's constant, T is temperature, and q is the electron charge. Ignoring the temperature dependence of R, dAV/dT is proportional to dIEE/dT-dT/dT, and dVOUT/dT is proportional to dICC/dT. Therefore, in order to minimize the variation of the gain AV and the output voltage VOUT over temperature, IEE must be proportional to temperature, while ICC must be constant.
Typically, this is not possible due to the fact that ICC=IEE (ignoring base current errors). Thus, a designer is typically forced to choose to maintain either constant gain AV or constant output voltage VOUT over temperature by appropriately defining the temperature dependence of IEE. A proportional-to-absolute-temperature (PTAT) current reference can be used to define IEE and ICC to maintain a constant gain AV over temperature, or a bandgap (BGAP) current reference can be used to define ICC and IEE to maintain a constant output voltage VOUT over temperature.
There have been various attempts to optimize performance characteristics of amplifiers. Some of these attempts have been directed at optimizing the gain by injecting a current at the amplifier output. For example, U.S. Pat. No. 5,798,660 describes how the gain of a CMOS differential amplifier stage may be enhanced by the injection of additional current at the drains of the differential pair. U.S. Pat. No. 5,436,594 describes a current source that biases a differential amplifier to control the gain of the amplifier.
None of these past attempts have independently optimized the temperature dependency of multiple parameters, such as gain and output voltage.
There is thus a need for a technique for amplifier output biasing that simultaneously minimizes temperature variations of various performance parameters.